Previous studies have discussed the measurement system for brackets in Song building standards (Yingzao fashi). However, still unclear is the origin of linggong length (uppermost parallel bracket in a Song bracket set, sometimes translated as “zero-additions bracket”). This paper re-examines this issue from a broader perspective, focusing on the relation between the form of dou (loadbearing block) and the wood grain on the frontal surface, in order to clarify the guiding principles behind the formation of bracket length in Yingzao fashi.